Sheet molding compounds (SMC) have heretofore been made. An important area of use is in the manufacturing of automotive body parts. Typically, SMC's are prepared by mixing unsaturated polyesters with styrene, mineral fillers, low shrinkage (or low profile) resins, free-radical initiators and other additives. The resulting SMC pastes must be initially fluid enough to incorporate fillers such as chopped glass fibers needed for reinforcement. Good wetting of the glass fibers is essential for satisfactory adhesion of the cured molding compounds to the glass fiber and overall product properties. The viscosity of the molding compound should initially be low, for example below 100,000 centipoises and for the molding operation should be high, as for example at least 2.times.10.sup.7 centipoises. The surface of the SMC paste should generally be tack-free. Current technology achieves maturation of the SMC paste based upon incorporation of MgO or Mg(OH).sub.2, which forms ionic bonds with the carboxyl terminated polyester resins. This route, however, has several disadvantages: (1) maturation times can be several days, (2) initial maturation rates and the final viscosities reached are very sensitive to moisture content of the pastes, (3) the maturation process continues and the pastes become too stiff and unusable for molding.
Various processes have been proposed to remedy the above problems. For example, Magrans U.S. Pat. No. 4,296,020 relates to ethylenically unsaturated monomer solutions of ethylenically unsaturated polyester, polyester urethane and polyisocyanurate resins which are quickly converted to moldable gels by the homogeneous formation of polyurea resins through the reaction of aliphatic polyamines and polyisocyanates. Although viscosity increases are achieved, such a system is generally not workable for mineral-filled and glass fiber reinforced unsaturated polyester based SMC. Moreover, viscosity buildup utilizing aliphatic diamines is too fast to permit sufficient fiberglass wetting. The use of conventional aromatic diamines are generally ineffective due to low solubility in the paste.
The polyamines utilized in U.S. Pat. No. 4,296,020 are different from the diamines of the present invention. In the Magrans patent, aliphatic polyamines are preferred. Such compounds have been found not to be workable for the maturation and molding of SMC compounds of the present invention, because: (1) they react too fast with di- or polyisocyanates leaving insufficient time for proper wetting of the glass fiber; a low viscosity induction period of at least five to ten minutes is needed;
(2) they are not effective or efficient in achieving high paste viscosities required for molding (&gt;2.times.10.sup.7 cps); and
(3) they are not inert toward the unsaturated polyester resins used in SMC's (e.g., see examples);, i.e., they undergo so-called "Michael additions" to the activated C-C double bonds of the polyesters; therefore, they cannot be premixed with the polyesters.
ICI U.S. Pat. Nos. 4,073,828 and 4,129,641 relate to the thickening of ethylenically unsaturated monomer solutions of unsaturated polyesters by first capping the hydroxyl terminated polyesters with diisocyanates and subsequently extending and/or crosslinking these prepolymers by the addition of diamines. Such a method is rather cumbersome and premature crosslinking is a problem as it effects moldability and final properties adversely.